Skip to main content

Full text of "Handbook Of Chemical Engineering - I"

See other formats


POWER GENERATION AND TRANSMISSION                   17
weight of air per pound of coal; To = temperature of gases leaving boiler; Ts = temperature of steam in boiler, Ti = temperature of air and coal supplied, the amount of this loss, per pound of coal, is (nearly) 0.24 (A + 1) (To — Ti) B.t.u. Of this amount, 0.24 (A + 1) (To — Ts) is an unnecessary loss. It is reduced by keeping TQ low: i.e., by using ample HS or an economizer: and by decreasing A close to the ideal value, which for pure-carbon fuel is 11.6 Ib. Too close an approach to this value leads to other losses, due to the formation of CO*
With minimum air supply, the percentage of CO2 in the flue gases (the amount of which is constant) is a maximum. Hence, the significance of C02 determination as an index of boiler efficiency: 16 per cent by volume represents remarkably good operation. The sum of COa, CO and O in the gases will almost invariably exceed 19 per cent. Air supply so restricted or so badly distributed as to lead to the presence of even small percentages of CO is highly wasteful: even 0.5 per cent may imply a 5 per cent loss of fuel.
Boiler Feed Waters.—Impurities in boiler waters not only reduce efficiency and capacity, but also impair quickness of response to demands for steam, increase the rate of deterioration of the boiler, and may produce dangerous conditions. The removal of deposits from a boiler nearly always involves considerable hard labor. Their prevention should be the aim in view. The unintelligent use of proprietary compounds is to be avoided. Some of them contain, besides reagents based on the water analysis, organic compounds which may loosen large masses of scale.
A water carrying 7 gr. of solids per gallon is generally considered fairly good, though much inferior waters must often be used, especially in the South and West. Even such a water will deliver 9 Ib. of solids per day to a 100-hp. boiler. These solids are generally more soluble in cold than in hot water. Hence, they are deposited in the boiler, causing mud or scale. Since carbonates precipitate at 212° and sulphates at about 300°, external heating of the feed water before it enters the boiler is sufficient to remove these common impurities. External chemical treatment is more commonly employed. The most successful treatment for waters containing both sulphates and bicarbonates (or carbonic acid) is that in which lime water and soda (hydrate or carbonate) are used together. Enough soda is used to break down the sulphates of lime and magnesia. Lime is added to absorb any remaining carbonic acid. The table on page 18 represents the practice of the L. M. Booth Co.
STEAM ENGINES
Classification.—Vertical engines are uncommon, though economical in floor space and (the type being otherwise fixed) likely to be more durable. The standard mill engine is horizontal, with releasing (Corliss) valve gear, which limits its speed to about 100 r.p.m. The double wristplate engine of this type is apt to be the more economical. Smaller engines may be "high-speed" (up to 300 r.p.m.) and should be of the four-valve or automatic cutoff type. Variable speed engines are occasionally used. A link gear is preferable to a throttle-governed engine in such cases. Piston speeds are rarely above 800 ft. per minute in any type. High-speed engines have strokes about equal to diameter: slow-speed engines may have strokes two or three times as great. Large engines should have "heavy-duty" (Tangye) frames, resting on the foundation for their entire length. Condensing engines should be used where economy is important if a water supply is available. The compound engine uses considerably less